The development of surgical techniques have made great progress over the years. For instance, for patients requiring brain surgery, non-invasive surgery is now available which is afflicted with very little trauma to the patient.
Stereotactic radiosurgery is such a minimally invasive treatment modality that allows delivery of a large single dose of radiation to a specific intracranial target while sparing surrounding tissue. Unlike conventional fractionated radiotherapy, stereotactic radiosurgery does not rely on, or exploit, the higher radiosensitivity of neoplastic lesions relative to normal brain (therapeutic ratio). Its selective destruction depends primarily on sharply focused high-dose radiation and a steep dose gradient away from the defined target. The biological effect is irreparable cellular damage and delayed vascular occlusion within the high-dose target volume. Because a therapeutic ratio is not required, traditionally radioresistant lesions can be treated. Because destructive doses are used, however, any normal structure included in the target volume is subject to damage.
One such non-invasive radiotherapy technique is so called LINAC (Linear Accelerator) radio therapy. In a LINAC radiotherapy system, a collimated x-ray beam is focused on a stereotactically identified intracranial target. In such an accelerator, electrons are accelerated to near light speed and are collided with a heavy metal, e.g. tungsten. The collision mainly produces heat but a small percentage of the energy is converted into highly energetic photons, which, because they are electrically produced, are called “x-rays”. The gantry of the LINAC rotates around the patient, producing an arc of radiation focused on the target. The couch in which the patient rests is then rotated in the horizontal plane, and another arc is performed. In this manner, multiple non-coplanar arcs of radiation intersect at the target volume and produce a high target dose, resulting in a minimal radiation affecting the surrounding brain. The x-rays are normally created by accelerating electrons to near light speed, and then colliding them with a heavy metal (e.g. tungsten). The collision mainly produces heat but a small percentage of the energy is converted to highly energetic protons, which are collimated and focus on the target.
Another system for non-invasive surgery is sold under the name of Leksell Gamma Knife®, which provides such surgery by means of gamma radiation. The radiation is emitted from a large number of fixed radioactive sources and are focused by means of collimators, i.e. passages or channels for obtaining a beam of limited cross section, towards a defined target or treatment volume. Each of the sources provides a dose of gamma radiation which is insufficient to damage intervening tissue. However, tissue destruction occurs where the radiation beams from all radiation sources intersect or converge, causing the radiation to reach tissue-destructive levels. The point of convergence is hereinafter referred to as the “focus point”. Such a gamma radiation device is referred to and described in U.S. Pat. No. 4,780,898.
In the system, the head of a patient is immobilized in a stereotactic instrument which defines the location of the treatment volume in the head. Further, the patient is secured in a patient positioning system which moves the entire patient so as to position the treatment volume in coincidence with the focus point of the radiation unit of the system.
Consequently, in radiotherapy systems, such as a LINAC system or a Leksell Gamma Knife® system, it is of a high importance that the positioning system which moves the patient so as to position the treatment volume in coincidence with the focus point of the radiation unit of the system is accurate and reliable. That is, the positioning system must be capable of position the treatment volume in coincidence with the focus point at a very high precision. Furthermore, this high precision must also be maintained over time.
A predetermined position of a positioning system in a radiation therapy system comprising a radiation therapy unit can be determined relatively a fixed radiation focus point of the radiation therapy unit by radiation measurements, e.g. using a phantom with radiation sensitive film provided in a certain position within the phantom. Another method is applying a radiation sensitive film on a tool adapted to be mounted in the positioning system, which tool is provided with reference marks such that it can be mounted in a defined position relatively the positioning system. According to a further method, a phantom with an ionization chamber provided in a certain position within the phantom is used. These indirect methods are however time-consuming and in-accurate.
Thus, there is a need of an efficient and reliable way of determining or verifying a predetermined position of the positioning system in a radiation therapy system comprising a radiation therapy unit relatively a fixed radiation focus point of the radiation therapy unit.